Using Composer with some text selected, I Insert->Link and press "Choose File..." and select an HTML document and press "Open" and nothing happens. It doesn't fill out the Link Location and does not enable the "OK" button. I am sad.

2.49.2 building will likely start soon. Mozilla didn't do backports for the two Fx 57.0.4 security fixes from last week yet. One does not need to be fixed. javascript.options.shared_memory is already false. But the timer fix should imho be included and might delay it a little.

Tried safe mode: Creating new web page does allow adding link as usual. After saving and opening again, I had the same problem -- insert link, choose existing HTML document for a link does not fill out that link.